Conductive buffer layers for semiconductor die assemblies and associated systems and methods

ABSTRACT

Conductive buffer layers for semiconductor die assemblies, and associated systems and methods are disclosed. In an embodiment, a semiconductor die assembly includes first and second semiconductor dies directly bonded to each other. The first semiconductor die includes a first copper pad and the second semiconductor die includes a second copper pad. The first and second copper pads form an interconnect between the first and second semiconductor dies, and the interconnect includes a conductive buffer material between the first and second copper pads, where the conductive buffer material includes aggregates of conductive particles. In some embodiments, the first and second copper pads are not conjoined but electrically connected to each other through the conductive buffer material. In some embodiments, the conductive buffer material is porous such that the aggregates of conductive particles can be compressed together in response to the pressure applied to the conductive buffer layer.

TECHNICAL FIELD

The present disclosure generally relates to semiconductor deviceassemblies, and more particularly relates to conductive buffer layersfor semiconductor die assemblies and associated systems and methods.

BACKGROUND

Semiconductor packages typically include one or more semiconductor dies(e.g., memory chips, microprocessor chip, imager chip) mounted on apackage substrate and encased in a protective covering. Thesemiconductor die may include functional features, such as memory cells,processor circuits, or imager devices, as well as bond pads electricallyconnected to the functional features. The bond pads can be electricallyconnected to corresponding conductive structures of the packagesubstrate, which may be coupled to terminals outside the protectivecovering such that the semiconductor die can be connected to higherlevel circuitry.

In some semiconductor packages, two or more semiconductor dies arestacked on top of each other to reduce the footprint of thesemiconductor packages. The semiconductor dies in the stack may bearranged in a pattern resembling stair-steps (which may be referred toas “shingle stacking”) such that a portion of the semiconductor dies maybe freely accessible - e.g., to attach bond wires to one or more bondpads located in the portion. In some cases, the semiconductor dies maybe stacked in a “zig-zag” pattern to increase a space above the bondpads with respect to a semiconductor die overlying above the bond padsso as to facilitate forming the bond wires. Such arrangements, however,tend to increase overall heights of the semiconductor packages. Further,the bond wires may add to the heights and/or introduce delays in signalpropagation.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the present technology can be better understood withreference to the following drawings. The components in the drawings arenot necessarily to scale. Instead, emphasis is placed on clearlyillustrating the overall features and the principles of the presenttechnology.

FIG. 1 illustrates various stages of process steps for direct bondingschemes.

FIGS. 2A-B and 3A-C illustrate schematic diagrams depicting stages ofprocesses forming semiconductor die assemblies in accordance withembodiments of the present technology.

FIG. 4 illustrates cross-sectional schematic diagrams of interconnectsfor semiconductor die assemblies in accordance with embodiments of thepresent technology.

FIG. 5 is a block diagram schematically illustrating a system includinga semiconductor die assembly in accordance with embodiments of thepresent technology.

FIG. 6 is a flowchart of a method of forming semiconductor dieassemblies in accordance with embodiments of the present technology.

DETAILED DESCRIPTION

Specific details of several embodiments of conductive buffer layers forsemiconductor die assemblies, and associated systems and methods aredescribed below. The term “semiconductor device or die” generally refersto a solid-state device that includes one or more semiconductormaterials. Examples of semiconductor devices (or dies) include logicdevices or dies, memory devices or dies, controllers, or microprocessors(e.g., central processing unit (CPU), graphics processing unit (GPU)),among others.

Such semiconductor devices may include integrated circuits orcomponents, data storage elements, processing components, and/or otherfeatures manufactured on semiconductor substrates. Further, the term“semiconductor device or die” can refer to a finished device or to anassembly or other structure at various stages of processing beforebecoming a finished functional device. Depending upon the context inwhich it is used, the term “substrate” may include a semiconductorwafer, a package substrate, an interposer, a semiconductor device ordie, or the like. Suitable steps of the methods described herein can beperformed with processing steps associated with fabricatingsemiconductor devices (wafer-level and/or die-level) and/ormanufacturing semiconductor packages.

Various computing systems or environments, e.g., high-performancecomputing (HPC) systems, require high bandwidth and low powerconsumption. Certain schemes of forming interconnects betweensemiconductor dies (e.g., a direct bonding scheme) may facilitatesatisfying the requirements, as well as providing form-factors suitablefor scaling physical dimensions (e.g., heights) of semiconductor dieassemblies of the HPC systems. The direct bonding scheme includesindividual conductive components (e.g., copper pads, conductive pads,bond pads) of a first semiconductor die (or a first wafer including thefirst semiconductor die) aligned and directly bonded to correspondingone of conductive components of a second semiconductor die (or a secondwafer including the second semiconductor die).

Further, a dielectric material surrounding each of the conductive padsof the first semiconductor die can be directly bonded to anotherdielectric material surrounding each of the conductive pads of thesecond semiconductor die. In other words, the bonding interface includestwo or more dissimilar materials of the first semiconductor die directlybonded to corresponding materials of the second semiconductor die (e.g.,between dielectric materials, between conductive materials) to forminterconnects and surrounding dielectric layers. As such, the directbonding scheme may also be referred to a combination bonding scheme, ahybrid bonding scheme, or the like.

In some embodiments, the conductive materials include copper (or othersuitable conductive materials or metals, such as tungsten, aluminum, orgold) as a primary constituent, and the dielectric materials includesilicon oxides (e.g., SiO₂), silicon nitrides (e.g., Si₃N₄), siliconcarbon nitrides (e.g., SiCN), silicon carbonates (e.g., SiCO), or thelike. During the direct bonding process, the dielectric materials of thefirst and second semiconductor dies (or the first and second wafersincluding the first and second semiconductor dies) are brought togethersuch that the dielectric materials adhere to each other and hermeticallyseal the conductive components aligned to each other.

Subsequently, the semiconductor dies are annealed at an elevatedtemperature (e.g., post bond annealing process) such that the conductivematerials of the conductive pads can expand - e.g., swell verticallytoward the bonding interface - at least partially due to the differencesin coefficients of thermal expansion (CTE) between the conductivematerials and the dielectric materials. This phenomenon may be referredto as CTE-based expansion of the conductive pads. Eventually, theconductive materials are conjoined to form permanent bonding betweenthem - e.g., metallurgical bonding. Additionally, the dielectricmaterials may enhance their bonding strength during the post bondannealing process. In some embodiments, the post bond annealing processcan be carried out at approximately 250° C. for 2 hours or so.

In some embodiments, the conductive pads have recessed surfaces withrespect to the surface of the dielectric materials. In this manner, whenthe dielectric materials of the semiconductor dies adhere to each other(e.g., prior to the post bond annealing process), bonding of thedielectric materials can be accomplished without any interference fromprotruded conductive pads. Further, the recess amount (e.g., a depth ofrecess, a recess depth, a dishing amount) for the conductive pads may bedevised and controlled to be within a certain range. The recess amountmay determine whether the conductive materials (e.g., copper) can besuitably conjoined to each other during the post bond annealing processwithout compromising the bonding strength or the interconnectcharacteristics.

For example, if the recess depths of the conductive pads areinsufficient (e.g., too shallow), the conductive materials, during theannealing process, may expand to pry open (e.g., pull apart, delaminate)the bonding interface between the dielectric materials, at least in theregion proximate to the conductive pads - e.g., resulting in oxideopenings. On the other hand, if the recess depths are excessive (e.g.,too deep), the conductive materials, during the annealing process, maynot be sufficiently conjoined to each other to form robustinterconnects - e.g., resulting in resistive copper joints or copperjoints openings.

Accordingly, controlling the recess depths may affect yield orreliability of the direct bonding process, and tends to impose strictrequirements to various process conditions and/or design factors. Forexample, conductive pads may be designed (e.g., laid out) to have widths(or lengths) of certain ranges to reduce variations in the recessdepths. In some embodiments, the conductive pads may be partitioned orbe surrounded with dummy pads to satisfy certain areal densityrequirements.

In some cases, elaborate targeting of the recess depths may be carriedout to determine suitable process conditions - e.g., over-polishtargeting during chemical-mechanical polish (CMP) process steps used togenerate the conductive pads. Even so, statistical random processvariations (e.g., life of CMP pads, variations in CMP slurries) maypresent challenges to the recess depth control. In some embodiments,variations in the recess depths need to be less than ± 5 nanometers (nm)across the entire wafer (e.g., across 300 mm diameter of the wafer). Thevariations in the recess depths may be referred to as a coplanarity ofthe conductive pads and maintaining the coplanarity of the conductivepads within a tolerable limit (e.g., within ± 5 nm) may increase costfor the semiconductor die assemblies.

The present technology provides for conductive buffer layers tofacilitate relaxing the recess depth requirements - i.e., widening thetolerable limits for the conductive pads coplanarity for hybrid bondingschemes. Such conductive buffer layers can be disposed between theconductive pads before two or more semiconductor dies are attached toeach other during the hybrid bonding process. In some embodiments, theconductive buffer layer is porous and includes conductive particles -e.g., copper particles, silver particles, gold particle, nickelparticles, or other suitable conductive nanoparticles. In someembodiments, the conductive particles have diameters of approximately 15nm to 50 nm.

In some embodiments, the conductive buffer layer is flexible (e.g.,malleable, squeezable, elastic, compressible, or otherwise adaptable)under pressure applied to the conductive buffer layer. In response tothe pressure applied to the conductive buffer layer (e.g., when theconductive pads expand into the conductive buffer layer responsive tothermal energy applied to the conductive pads), the conductive bufferlayer may deform (e.g., as a result of conductive particles transportingto regions with relatively less pressure). Additionally, oralternatively, the conductive particles may be brought closer to oneanother or be compressed together (e.g., migrating within the conductivebuffer layer) such that the conductive buffer layer occupies lessvolume. As such, the conductive buffer layer may become less porous as aresult of being subject to the pressure.

In some embodiments, in response to receiving thermal energy (e.g.,during the post bond annealing process at 250° C. for 2 hours) and/orbeing subject to the pressure, the conductive particles may coalesce(e.g., aggregate, merge, fuse, amalgamate) to form electricallyconductive structures (e.g., similar to sintered metal). In a particularembodiment, the post bond annealing process provides sintering for theconductive particles. The electrically conductive structures may beporous in their nature. In some embodiments, the electrically conductivestructures have porosity of approximately 30%, which is expected to haveno significant effect on the electrical conductivity. Moreover, theporosity of the electrically conductive structures may vary based on thethermal energy (e.g., post bond annealing temperature or time) and/orthe magnitude of pressure (e.g., due to variations in the recess depths)applied to the conductive particles. In some embodiments, theelectrically conductive structures includes the porosity of 30 ± 10%. Inother embodiments, the electrically conductive structures includes theporosity of less than 20%.

In view of the conductive buffer layer present between the upper andlower conductive pads to be directly bonded, the recess depths can betargeted to be relatively aggressive - e.g., generating generallygreater recess amounts when compared to the recess depths absent theconductive buffer layer. In this manner, when the conductive padsexpand, e.g., during the post bond annealing process, the conductivebuffer layer can provide a squeezable cushion (e.g., a buffer region,compressible room) against the rigid conductive pads bumping into eachother - e.g., when one or more conductive pads have recess depthsinadequately shallow tending to result in oxide openings without theconductive buffer layer. Alternatively, the conductive buffer layer canprovide an electrically conductive medium between the conductive padseven if the conductive pads do not physically connect to each otherafter completing the post bond annealing process - e.g., when one ormore conductive pads have recess depths inadequately deep tending toresult in resistive copper joints or copper joints openings without theconductive buffer layer.

As used herein, the terms "front," "back," "vertical," "lateral,""down," "up," "top," "bottom," "upper," and "lower" can refer torelative directions or positions of features in the semiconductor deviceassemblies in view of the orientation shown in the Figures. For example,“upper” or “uppermost” can refer to a feature positioned closer to thetop of a page than another feature. These terms, however, should beconstrued broadly to include semiconductor devices having otherorientations. Unless stated otherwise, terms such as “first” and“second” are used to arbitrarily distinguish between the elements suchterms describe. Thus, these terms are not necessarily intended toindicate temporal or other prioritization of such elements.

FIG. 1 illustrates various stages of process steps for direct bondingschemes. Diagram 100A illustrates a portion of a semiconductor die 101with a substrate 110 having integrated circuitry (not shown) and athrough-substrate via (TSV) 115 coupled with the integrated circuitry.In some embodiments, the TSV 115 includes a first conductive material117 (e.g., tungsten) and a conductive barrier layer 118 (e.g., TiN). Thesemiconductor die 101 also includes a conductive pad 125 (which may alsobe referred to as a bond pad) formed in a dielectric layer 120 (e.g.,silicon oxides, silicon nitrides, silicon carbon nitrides, siliconcarbonates, or a combination thereof). The conductive pad 125 iselectrically connected to the TSV 115 (and to the integrated circuitrythrough the TSV 115). In some embodiments, the conductive pad 125includes a second conductive material 127 (e.g., copper) and anotherconductive barrier layer 128 (e.g., TaN).

The conductive pad 125 depicted in diagram 100A includes a surfacerecessed by a depth D with respect to the surface of the dielectriclayer 120 (i.e., the surface opposite to the substrate 110). In someembodiments, CMP process steps are used to form the conductive pad 125,and the recess may be a result of the CMP process. For example, therecess may be formed during over-polishing steps that remove excessiveconductive material 127 on the surface of the dielectric layer 120.Moreover, the amount of recess (e.g., the recess depth D) may betargeted to ensure the surface of the conductive pad 125 not to protrudeabove the surface of the dielectric layer 120 - e.g., to avoid suchprotruded conductive pads 125 interfering with the bonding processdescribed with reference to diagram 100B. The amount of recess may betargeted to be within a certain range such that the conductive materials127 can form an interconnect 140 without compromising overall bondingintegrity as described with reference to diagram 100C.

Diagram 100B illustrates two semiconductor dies 101 a and 101 b (or twowafers including the semiconductor dies 101 a and 101 b) that areattached together such that dielectric materials of the topsemiconductor die 101 b and bottom semiconductor die 101 a adhere toeach other to form dielectric-to-dielectric bonding 130 at the bondinginterface 105. In some embodiments, the dielectric surfaces areactivated (e.g., using a plasma treatment process) to facilitate thebonding of the dielectric surfaces. Also, conductive pads (e.g., the topconductive pad 125 b and the bottom conductive pad 125 a) of the top andbottom semiconductor dies 101 a and 101 b are aligned to face each otherbut are not connected to each other due to the recessed surfaces of theconductive pads 125 a/b.

Diagram 100C illustrates that the bonded dies/wafers are annealed in anelevated temperature (e.g., around 400° C.) such that the conductivematerials of the top and bottom conductive pads 125 a/b may expandtoward each other in response to receiving thermal energy during thepost bond annealing process (e.g., due to the mismatch in CTEs betweenthe conductive materials and the dielectric materials) within the openspace defined by the recess surfaces and the dielectric materialsurrounding the conductive pads 125 a/b.

When the surfaces of the top and bottom conductive materials are incontact as a result of the CTE-based expansion, the conductive materialsare conjoined (e.g., via atomic migration (intermixing, diffusion) fromone conductive material to another conductive material) to formmetal-to-metal bonding 135 - e.g., metallurgical bonding, permanentbonding. Once the metallurgical bonding is formed between the conductivepads 125 a/b (thus, forming the interconnect 140), the conductivematerials do not separate (or sever) when the bonded dies/wafers arebrought to the ambient temperature or operating temperatures of thesemiconductor die assemblies. In this manner, the bonding interface 105includes the dielectric-to-dielectric bonding 130 and the metal-to-metalbonding 135.

FIGS. 2A-B illustrate schematic diagrams 200A through 200D depictingstages of a process forming semiconductor die assemblies in accordancewith embodiments of the present technology. Diagram 200A illustrates aportion of a semiconductor die 201, which may be an example of orinclude aspects of the semiconductor die 101 described with reference toFIG. 1 . For example, the semiconductor die 201 includes the substrate110 having integrated circuitry (not shown) and the dielectric layer120.

The semiconductor die 201 also includes one or more conductive pads 225formed in the dielectric layer 120. The conductive pads 225 may beexamples of or include aspects of the conductive pads 125 described withreference to FIG. 1 . The TSVs 115 coupling the conductive pads 225 tothe integrated circuitry are omitted in FIGS. 2A-B. The conductive pad225 has a surface 230 recessed with respect to a surface 235 of thedielectric layer 120 (the amount of recess denoted as D) - e.g., as aresult of the CMP process described with reference to FIG. 1 . As shownin diagram 200A, the surface of the conductive pad 225 is uncovered(i.e., exposed) at this stage of the process.

Diagram 200B illustrates a conductive buffer layer 210 formed on theconductive pads 225. Also illustrated in diagram 200B are an optionalsacrificial layer 215 between the conductive pads 225 and the conductivebuffer layer 210 and an optional mask 237 on the surface 235 of thedielectric layer 120. In some embodiments, the conductive buffer layer210 adheres to the sacrificial layer 215. As such, the sacrificial layer215 may be regarded as a tacky layer (or an adhesion layer) for theconductive buffer layer 210. In some embodiments, the sacrificial layer215 includes flux that protects the surface 230 of the conductive pads225 from oxidation. In some embodiments, the sacrificial layer 215 isconfigured to decompose at an elevated temperature - e.g., at atemperature of about 100° C. or greater. In some embodiments, thesacrificial layer 215 forms only on the conductive pads 225 using maskeddeposition process steps - e.g., using a mask layer (e.g., a photoresistlayer, a hard-mask layer) on the surface 235 of the dielectric layer120, which includes openings corresponding to the conductive pads 225.

In some embodiments, the conductive buffer layer 210 may be formed onthe conductive pads 225 based on depositing conductive particles (e.g.,copper particles) over the conductive pads 225 while blocking theconductive particles from depositing on the surface 235 of thedielectric layer 120 unoccupied by the conductive pads 225. In someembodiments, screen-printing techniques is used to deposit theconductive particles over the conductive pads 225 only. In otherembodiments, masked deposition techniques is used to deposit theconductive particles over the conductive pads 225 only. In someembodiments, the conductive particles deposited over the conductive pads225 may be sintered at an elevated temperature (e.g., 100° C. orgreater).

In some embodiments, the conductive buffer layer 210 may be formed onthe conductive pads 225 based on selectively sintering conductiveparticles that have been disposed over the conductive pads 225. Forexample, a layer of liquid including conductive particles is coated onthe semiconductor die 201 (e.g., on the surface 230 of the conductivepads 225 and on the surface 235 of the dielectric layer 120).Subsequently, precision electromagnetic radiation (e.g., laser) may beused to selectively sinter the conductive particles on the conductivepads 225 only. Thereafter, the un-sintered conductive particles areremoved (e.g., from the surface 235 of the dielectric layer 120).

In some embodiments, the conductive buffer layer 210 may be formed onthe conductive pads 225 based on selectively spraying a solutionincluding conductive particles (e.g., copper nanoparticle ink) over theconductive pads 225. For example, the copper nanoparticle ink can bedirectly sprayed on the surface 230 of the conductive pads 225 using aprecision nozzle. Subsequently, the copper nanoparticle ink depositedover the conductive pads 225 may be sintered at an elevated temperature(e.g., 100° C. or greater).

In some embodiments, while the conductive buffer layer 210 is formed onthe conductive pads 225, the optional mask 237 (e.g., a photoresist, ahard-mask of organic and/or inorganic materials) is deployed on thesurface 235 of the dielectric layer 120. After the conductive bufferlayer 210 has been formed only on the conductive pads 225, the mask 237is removed. In this manner, the surface 235 of the dielectric layer 120can be protected during the various process steps associated withforming the conductive buffer layer 210 on the conductive pads 225.

Further, during the sintering process steps, the conductive particlesmay be protected from oxidation. For example, oxygen pumping (tominimize oxygen partial pressure in the ambient) may be used during thesintering process. In some instances, the sintering may be carried outin a H₂ ambient. In some cases, oxides formed on the surface ofconductive particles can be reduced (or removed) using formic acidcleaning.

Diagram 200C illustrates two semiconductor dies 201 (also identifiedindividually as semiconductor dies 201 a and 201 b) that are attachedtogether to form the bonding interface 205 as described with referenceto FIG. 1 (e.g., diagram 100B). In a particular aspect of an embodimentshown in diagram 200C, the lower (bottom) semiconductor die 201 aincludes the conductive buffer layer 210 and the optional sacrificiallayer 215 while the upper (top) semiconductor die 201 b does not. In afurther particular aspect of this embodiment, the surface 235 a ofdielectric layer 120 a of the semiconductor die 201 a is in contact withthe surface 235 b of dielectric layer 120 b of the semiconductor die 201b forming the dielectric-to-dielectric bonding at the bonding interface205.

As shown in diagram 200C, two semiconductor dies 201 a and 201 b arearranged such that the conductive pads 225 (also identified individuallyas conductive pads 225 a/b) of the semiconductor dies 201 a and 201 bare aligned to each other - e.g., the conductive pad 225 a aligned tothe conductive pad 225 b. Subsequently, the semiconductor dies 201 a and201 b (or the wafers including the semiconductor dies 201 a and 201 b)are brought to an elevated temperature (e.g., post bond annealingprocess steps) to facilitate the CTE-based expansion of the conductivepads 225.

Diagram 200D illustrates that interconnects 240 are formed - e.g., as aresult of the post bond annealing process. In other words, theconductive pads 225 a and 225 b expand upon receiving thermal energy atthe elevated temperature (e.g., during the post bond annealing process)such that the surfaces 230 of the conductive pads 225 a and 225 badvance toward the bonding interface 205. Moreover, the sacrificiallayer 215 may decompose subject to the thermal energy (e.g., during thepost bond annealing process). Accordingly, the sacrificial layer 215 isomitted in the diagram 200D.

In some embodiments, the conductive buffer layer 210 allows a widerrange of variations in the recess depths - e.g., widening a tolerablelimit for the coplanarity of the conductive pads 225. For example, thecoplanarity of the conductive pads 225 may be required to be within ± 5nm absent the conductive buffer layer 210. With the conductive bufferlayer 210 included in at least one of the semiconductor dies 201 a/b,the tolerable limit for the coplanarity of the conductive pads 225 maybe widened (e.g., increased, relaxed) to a greater value, such as ± 7nm, ± 10 nm, ± 15 nm, or even greater.

As a result of using the conductive buffer layer 210, the interconnects240 include a conductive buffer material (i.e., conductive buffermaterial included in the conductive buffer layer 210) in addition to theconductive pads 225 - e.g., between the conductive pads 225 a/b. In someembodiments, the conductive buffer layer is less dense than the upperand lower conductive pads 225 (e.g., copper pads formed byelectroplating process steps). Although the diagram 200D illustrates theconductive buffer layer 210 generally in a plane corresponding to thebonding interface 205, locations of the conductive buffer layer 210 mayvary with respect to the bonding interface 205 in view of varying recessdepths of the conductive pads 225 as described in more detail withreference to FIG. 4 . Further, the conductive buffer layer 210 may bediscontinuous at certain locations - e.g., the interconnects 240 mayinclude a portion where the conductive pads 225 from the lower and uppersemiconductor dies 201 a/b forming direct metal-to-metal bonding.

Although the foregoing example schematic diagrams illustrate only one ofsemiconductor dies 201 a or 201 b having the conductive buffer layer210, the present technology is not limited thereto. For example, both ofthe semiconductor dies 201 a and 201 b can include the conductive bufferlayer 210.

FIGS. 3A-C illustrate schematic diagrams 300A through 300E depictingstages of a process forming semiconductor die assemblies in accordancewith embodiments of the present technology. Diagram 300A illustrates thesemiconductor die 201 a after the conductive buffer layer 210 a has beenformed on the conductive pads 225 as described with reference to thediagram 200B. In some embodiments, the semiconductor die 201 a is thesame type of semiconductor die as the semiconductor dies (e.g., memorydies) that the semiconductor die 201 a carries (e.g., semiconductor die201 b, semiconductor dies 301 a/b). In other embodiments, thesemiconductor die 201 a is an interface die.

The interface die may be different types of semiconductor dies (e.g.,logic dies, controller dies) than the semiconductor dies that thesemiconductor die 201 a carries. In some embodiments, a logic die isconfigured to exchange electrical signals with the semiconductor dies itcarries and with higher level circuitry (e.g., a host device) coupledwith the logic die. In some embodiments, the interface die is aninterposer die having various conductive structures (e.g.,redistribution layers, vias, interconnects) configured to routeelectrical signals between the semiconductor dies it carries and higherlevel circuitry - e.g., a central processing unit (CPU) coupled with theinterposer die.

Diagram 300A also illustrates a semiconductor die 301 a, which may be anexample of or include aspects of the semiconductor die 201 b. Forexample, the semiconductor die 301 a includes the substrate 110including integrated circuitry (not shown). Further, the semiconductordie 301 a includes dielectric layers 120 c/d formed on both sides of thesubstrate 110. The semiconductor die 301 a may be referred to as amiddle die of a stack of semiconductor dies. The semiconductor die 301 aincludes conductive pads 225 (also identified individually as conductivepads 225 c and 225 d) in both of the dielectric layers 120 c/d such thatthe semiconductor die 301 a can be attached to a semiconductor dieunderneath (e.g., the semiconductor die 201 a) and a semiconductor dieabove (e.g., semiconductor die 301 b as shown in diagrams 300C/D).

Diagram 300B illustrates the semiconductor die 301 a has been directlyattached to the semiconductor die 201 a - e.g., the dielectric layer 120a of semiconductor die 201 a and the dielectric layer 120 c of thesemiconductor die 301 a form dielectric-to-dielectric bonding at thebonding interface 205 a. Subsequently, the conductive buffer layer 210 bis formed on the conductive pads 225 d of the semiconductor die 301 a.Also illustrated in diagram 300B is the optional sacrificial layer 215b.

Diagram 300C illustrates that another semiconductor die 301 b, which maybe an example of or include aspects of the semiconductor die 201 b. Forexample, the semiconductor die 301 b includes the substrate 110 bincluding integrated circuitry (not shown) and a dielectric layer 120 e.The semiconductor die 301 b may be referred to as a top die of a stackof semiconductor dies. The semiconductor die 301 b includes conductivepads 225 (also identified individually as conductive pads 225 e) in itsdielectric layer 120. The substrate 110 b may be thicker than thesubstrates of the middle semiconductor dies.

Diagram 300D illustrates the semiconductor die 301 b has been directlyattached to the semiconductor die 301 a - e.g., the dielectric layer 120e of semiconductor die 301 b and the dielectric layer 120 d of thesemiconductor die 301 a form another dielectric-to-dielectric bonding atthe bonding interface 205 b. Subsequently, the stack of semiconductordies 201 a, 301 a, and 301 b can be brought to an elevated temperature(e.g., post bond annealing process steps) to have the conductive pads225 of all semiconductor dies (e.g., semiconductor dies 201 a, 301 a,and 301 b) expand (e.g., during the post bond annealing process) to forminterconnects. Moreover, the sacrificial layer 215 may decompose at theelevated temperature.

Diagram 300E illustrates a semiconductor die assembly 370 including thesemiconductor dies 201 a, 301 a, and 301 b. As a result of the post bondannealing process, the conductive pads 225 from the upper and lowersemiconductor dies conjoin to form interconnects 240 (also identifiedindividually as interconnects 240 a/b) between the semiconductor dies.Each of the interconnects 240 includes a conductive buffer material(e.g., the conductive buffer material of the conductive buffer layer210) in addition to the conductive pads (e.g., between the conductivepads) from the upper and lower semiconductor dies. The optionalsacrificial layers 215 a/b is no longer included in the diagram 300E asthey have decomposed during the post bond annealing process as describedabove. Although the semiconductor die assembly 370 as shown in thediagram 300E includes one middle semiconductor die 301 a, the presenttechnology is not limited thereto. For example, in other embodiments,the semiconductor die assembly 370 includes two or more middlesemiconductor dies 301 a - e.g., four (4), eight (8), twelve (12), oreven greater quantity of middle semiconductor dies.

FIG. 4 illustrates cross-sectional schematic diagrams 400A through 400Eof interconnects 440 a-e for semiconductor die assemblies in accordancewith embodiments of the present technology. The interconnects 440 a-emay be examples of or include aspects of interconnects 140 or 240described with reference to FIGS. 1 through 3 . For example, each of theinterconnects 440 a-e includes the conductive buffer layer 210 (alsoidentified individually as conductive buffer layer 210 a-e) (or theconductive buffer material of the conductive buffer layer 210) inaddition to conductive pads 225 (also identified individually as upperconductive pads 225-1 and lower conductive pads 225-2) from the top(upper) semiconductor die (e.g., the semiconductor die 201 b, thesemiconductor die 301 b) and the bottom (lower) semiconductor die (e.g.,the semiconductor die 201 a) with respect to the bonding interface 205,respectively.

Diagram 400A depicts the interconnect 440 a including the conductivebuffer layer 210 a located generally at the bonding interface 205.Recess depths of the conductive pads 225-½ may have been comparable toeach other such that the CTE-based expansion of the conductive pads225-½ brings the conductive buffer layer 210 a approximately at thebonding interface 205. As depicted in the diagram 400A, the surface ofconductive pad 225-1 is separate from the surface of conductive pad225-2 but the conductive buffer layer 210 a electrically couples theconductive pad 225-1 with the conductive pad 225-2 to avoid theinterconnect 440 a being highly resistive or electrically open.

Diagram 400B depicts the interconnect 440 b including the conductivebuffer layer 210 b located slightly off of the bonding interface 205. Atleast partially due to the process variations in generating theconductive pads 225-½, the recess depths of the conductive pads 225-½may have been dissimilar (e.g., the conductive pad 225-2 has a shallowerrecess depth than that of the conductive pad 225-1). As a result of thevariations in recess depth of the conductive pads 225-½, the CTE-basedexpansion of the conductive pads 225-½ may bring the conductive bufferlayer 210 b off of the bonding interface 205.

Diagram 400C depicts the interconnect 440 c including the conductivebuffer layer 210 c that may be discontinuous at certain regions. Inother words, at least a portion of the surface of the conductive pad225-1 is conjoined to at least another portion of the surface of theconductive pad 225-2 - e.g., at least partial metal-to-metal bondingoccurred between the conductive pads 225-1 and 225-2. Although theconductive buffer layer 210 c is shown generally at the bondinginterface 205, the conductive buffer layer 210 c may be located off ofthe bonding interface 205 in view of the variations in recess depth ofthe conductive pads 225-½.

Diagram 400D depicts the interconnect 440 d including the conductivebuffer layer 210 d located below the bonding interface 205. Further, theinterconnect 440 d includes the conductive pad 225-1 extending past thebonding interface 205. The conductive pad 225-1 may represent aconductive pad protruded above the surface of the dielectric layer, inwhich the conductive pad is formed - e.g., after completing the CMPprocess generating the conductive pad. Such a protruded conductive padmay be regarded as an extreme incoming condition for the hybrid bondingprocess, which tends to hinder forming a robust hybrid bonding interfacewithout the present technology.

Diagram 400D, however, demonstrates that the present technologyfacilitates overcoming challenges associated with such an extremecondition. For example, the corresponding conductive pad 225-2 has asuitable recess depth to accommodate the protruded conductive pad225-1 - e.g., in view of the widened tolerable limits for the conductivepad coplanarity. Further, the conductive buffer layer 210 d can providea squeezable cushion during the CTE-based expansion of the conductivepads 225-½. Accordingly, the interconnect 440 d is expected to havesimilar characteristics as other interconnects (e.g., interconnects 440a-c) in spite of the protruded conductive pad 225-1.

Diagram 400E depicts the interconnect 440 e generally similar to theinterconnect 440 d. Further, similar to the conductive buffer layer 210c, the conductive buffer layer 210 e of the interconnect 440 e may bediscontinuous at certain regions. In other words, at least a portion ofthe surface of the conductive pad 225-1 is conjoined to at least anotherportion of the surface of the conductive pad 225-2 - e.g., at leastpartial metal-to-metal bonding occurred between the conductive pads225-1 and 225-2.

FIG. 5 is a block diagram schematically illustrating a system 500including a semiconductor die assembly in accordance with embodiments ofthe present technology. The system 500 can include a semiconductordevice assembly 570, a power source 572, a driver 574, a processor 576,and/or other subsystems or components 578. The semiconductor deviceassembly 570 can be incorporated into any of a myriad of larger and/ormore complex systems, a representative example of which is the system500 shown schematically in FIG. 5 . The semiconductor die assembly 370described with reference to FIG. 5 may be included in the semiconductordevice assembly 570 of the system 500.

The semiconductor device assembly 570 can have features generallysimilar to the semiconductor die assembly 370 described above withreference to FIGS. 3A-C. For example, the semiconductor device assembly570 includes two or more semiconductor dies (e.g., the semiconductordies 201 a, 301 a, and 301 b) that are directly bonded to each other.Further, the semiconductor device assembly 570 includes interconnectsformed between the semiconductor dies. Each of the interconnects mayinclude a conductive buffer material in addition to the first and secondconductive pads (e.g., between the first and second conductive pads)from the top and bottom semiconductor dies, respectively.

In some embodiments, the semiconductor device assembly 570 includes afirst semiconductor die including a first semiconductor substrate, afirst dielectric layer over the first semiconductor substrate, and afirst copper pad in the first dielectric layer, the first copper padhaving a first surface opposite to the first semiconductor substrate.Further, the semiconductor device assembly 570 includes a secondsemiconductor substrate, a second dielectric layer over the secondsemiconductor substrate, and a second copper pad in the seconddielectric layer, the second copper pad having a second surface oppositeto the second semiconductor substrate. Moreover, the first dielectriclayer is in direct contact with the second dielectric layer at a bondinginterface between the first and second semiconductor dies, and the firstand second copper pads form an interconnect between the first and secondsemiconductor dies, the interconnect having a conductive buffer materialbetween the first and second copper pads. In some embodiments, theconductive buffer layer includes aggregates of conductive particles. Insome embodiments, the conductive buffer layer is less dense than thefirst and second copper pads.

In some embodiments, the conductive buffer material is porous andincludes copper particles. In some embodiments, at least a first portionof the first surface is conjoined to at least a second portion of thesecond surface. In some embodiments, the first surface is separate fromthe second surface, and where the first copper pad is electricallyconnected to the second copper pad through the conductive buffermaterial. In some embodiments, either the first surface or the secondsurface extends past the bonding interface.

The resulting system 500 can perform any of a wide variety of functions,such as memory storage, data processing, and/or other suitablefunctions. Accordingly, representative systems 500 can include, withoutlimitation, hand-held devices (e.g., mobile phones, tablets, digitalreaders, and digital audio players), computers, and appliances.Components of the system 500 may be housed in a single unit ordistributed over multiple, interconnected units (e.g., through acommunications network). The components of the system 500 can alsoinclude remote devices and any of a wide variety of computer readablemedia.

FIG. 6 is a flowchart 600 of a method of forming semiconductor dieassemblies in accordance with embodiments of the present technology. Theflowchart 600 may include aspects of methods as described with referenceto FIGS. 1 through 4 .

The method comprises providing a first semiconductor die including afirst dielectric layer, where the first dielectric layer includes afirst bond pad having a first top surface recessed with respect to afirst surface of the first dielectric layer, and where a conductivebuffer layer is disposed on the bond pad, the conductive buffer layerbeing malleable to deform in response to pressure applied to theconductive buffer layer (box 610). The method further comprisesproviding a second semiconductor die including a second dielectric layerhaving a second surface, where the second dielectric layer includes asecond bond pad having a second top surface (box 615). The methodfurther comprises attaching the first and second semiconductor dies suchthat the first surface is in contact with the second surface to form abonding interface and the first bond pad is aligned to and facing thesecond bond pad (box 620). The method further comprises heating thefirst and second semiconductor dies attached to each other (box 625).

In some embodiments, a sacrificial layer is disposed between the firsttop surface of the first bond pad and the conductive buffer layer, andthe conductive buffer layer adheres to the sacrificial layer. In someembodiments, heating the first and second semiconductor dies attached toeach other includes decomposing the sacrificial layer at a temperaturegreater than 100° C. In some embodiments, the second top surface of thesecond bond pad is recessed with respect to the second surface of thesecond dielectric layer, and both the first and second top surfaces ofthe first and second bond pads expand toward the bonding interface inresponse to heating the first and second semiconductor dies.

In some embodiments, at least a first portion of the first top surfaceis conjoined to at least a second portion of the second top surface as aresult of the first and second bond pads expanding toward the bondinginterface. In some embodiments, the first top surface is separate fromthe second top surface after heating the first and second semiconductordies, and the first bond pad is electrically connected to the secondbond pad through the conductive buffer layer. In some embodiments, thesecond top surface of the second bond pad is protruded with respect tothe second surface of the second dielectric layer, and both the firstand second top surfaces of the first and second bond pads expand towardeach other in response to heating the first and second semiconductordies.

In some embodiments, at least a first portion of the first top surfaceis conjoined to at least a second portion of the second top surface as aresult of the first and second bond pads expanding toward each other. Insome embodiments, the first top surface is separate from the second topsurface, and the first bond pad is electrically connected to the secondbond pad through the conductive buffer layer.

It should be noted that the methods described above describe possibleimplementations, and that the operations and the steps may be rearrangedor otherwise modified and that other implementations are possible.Furthermore, embodiments from two or more of the methods may becombined. From the foregoing, it will be appreciated that specificembodiments of the technology have been described herein for purposes ofillustration, but that various modifications may be made withoutdeviating from the disclosure. In addition, while in the illustratedembodiments certain features or components have been shown as havingcertain arrangements or configurations, other arrangements andconfigurations are possible. Moreover, certain aspects of the presenttechnology described in the context of particular embodiments may alsobe combined or eliminated in other embodiments.

The devices discussed herein, including a semiconductor device, may beformed on a semiconductor substrate or die, such as silicon, germanium,silicon-germanium alloy, gallium arsenide, gallium nitride, etc. In somecases, the substrate is a semiconductor wafer. In other cases, thesubstrate may be a silicon-on-insulator (SOI) substrate, such assilicon-on-glass (SOG) or silicon-on-sapphire (SOS), or epitaxial layersof semiconductor materials on another substrate. The conductivity of thesubstrate, or sub-regions of the substrate, may be controlled throughdoping using various chemical species including, but not limited to,phosphorous, boron, or arsenic. Doping may be performed during theinitial formation or growth of the substrate, by ion-implantation, or byany other doping means.

As used herein, including in the claims, “or” as used in a list of items(for example, a list of items prefaced by a phrase such as “at least oneof” or “one or more of”) indicates an inclusive list such that, forexample, a list of at least one of A, B, or C means A or B or C or AB orAC or BC or ABC (i.e., A and B and C). Also, as used herein, the phrase“based on” shall not be construed as a reference to a closed set ofconditions. For example, an exemplary step that is described as “basedon condition A” may be based on both a condition A and a condition Bwithout departing from the scope of the present disclosure. In otherwords, as used herein, the phrase “based on” shall be construed in thesame manner as the phrase “based at least in part on.” The term“exemplary” used herein means “serving as an example, instance, orillustration,” and not “preferred” or “advantageous over otherexamples.”

From the foregoing, it will be appreciated that specific embodiments ofthe invention have been described herein for purposes of illustration,but that various modifications may be made without deviating from thescope of the invention. Rather, in the foregoing description, numerousspecific details are discussed to provide a thorough and enablingdescription for embodiments of the present technology. One skilled inthe relevant art, however, will recognize that the disclosure can bepracticed without one or more of the specific details. In otherinstances, well-known structures or operations often associated withmemory systems and devices are not shown, or are not described indetail, to avoid obscuring other aspects of the technology. In general,it should be understood that various other devices, systems, and methodsin addition to those specific embodiments disclosed herein may be withinthe scope of the present technology.

What is claimed is:
 1. A semiconductor die, comprising: a semiconductorsubstrate; a dielectric layer over the semiconductor substrate; a bondpad in the dielectric layer, the bond pad including a top surface thatis recessed with respect to a surface of the dielectric layer oppositeto the semiconductor substrate; and a conductive buffer layer on thebond pad, wherein the conductive buffer layer includes aggregates ofconductive particles, and is flexible to deform in response to pressureapplied to the conductive buffer layer.
 2. The semiconductor die ofclaim 1, further comprising: a sacrificial layer between the top surfaceof the bond pad and the conductive buffer layer, wherein the conductivebuffer layer adheres to the sacrificial layer.
 3. The semiconductor dieof claim 2, wherein the sacrificial layer is configured to decompose ata temperature greater than 100° C.
 4. The semiconductor die of claim 1,wherein the conductive buffer layer is porous such that the aggregatesof conductive particles are compressed together in response to thepressure applied to the conductive buffer layer.
 5. The semiconductordie of claim 4, wherein the conductive particles includes copper.
 6. Thesemiconductor die of claim 1, wherein the conductive buffer layer isformed based on at least one of: depositing conductive particles overthe bond pad while blocking the conductive particles from depositing onthe surface of the dielectric layer unoccupied by the bond pad;selectively sintering conductive particles that have been disposed overthe bond pad; or selectively spraying a solution including conductiveparticles over the bond pad.
 7. A method, comprising: providing a firstsemiconductor die including a first dielectric layer, wherein the firstdielectric layer includes a first bond pad having a first top surfacerecessed with respect to a first surface of the first dielectric layer,and wherein a conductive buffer layer is disposed on the bond pad, theconductive buffer layer including aggregates of conductive particles andbeing flexible to deform in response to pressure applied to theconductive buffer layer; providing a second semiconductor die includinga second dielectric layer having a second surface, wherein the seconddielectric layer includes a second bond pad having a second top surface;attaching the first and second semiconductor dies such that the firstsurface is in contact with the second surface to form a bondinginterface and the first bond pad is aligned to and facing the secondbond pad; and heating the first and second semiconductor dies attachedto each other.
 8. The method of claim 7, wherein a sacrificial layer isdisposed between the first top surface of the first bond pad and theconductive buffer layer, and wherein the conductive buffer layer adheresto the sacrificial layer.
 9. The method of claim 8, wherein heating thefirst and second semiconductor dies attached to each other comprises:decomposing the sacrificial layer at a temperature greater than 100° C.10. The method of claim 7, wherein the second top surface of the secondbond pad is recessed with respect to the second surface of the seconddielectric layer, and wherein both the first and second top surfaces ofthe first and second bond pads expand toward the bonding interface inresponse to heating the first and second semiconductor dies.
 11. Themethod of claim 10, wherein at least a first portion of the first topsurface is conjoined to at least a second portion of the second topsurface as a result of the first and second bond pads expanding towardthe bonding interface.
 12. The method of claim 10, wherein the first topsurface is separate from the second top surface after heating the firstand second semiconductor dies, and wherein the first bond pad iselectrically connected to the second bond pad through the conductivebuffer layer.
 13. The method of claim 7, wherein the second top surfaceof the second bond pad is protruded with respect to the second surfaceof the second dielectric layer, and wherein both the first and secondtop surfaces of the first and second bond pads expand toward each otherin response to heating the first and second semiconductor dies.
 14. Themethod of claim 13, wherein at least a first portion of the first topsurface is conjoined to at least a second portion of the second topsurface as a result of the first and second bond pads expanding towardeach other.
 15. The method of claim 13, wherein the first top surface isseparate from the second top surface, and wherein the first bond pad iselectrically connected to the second bond pad through the conductivebuffer layer.
 16. A semiconductor die assembly, comprising: a firstsemiconductor die including: a first semiconductor substrate; a firstdielectric layer over the first semiconductor substrate; and a firstcopper pad in the first dielectric layer, the first copper pad having afirst surface opposite to the first semiconductor substrate; and asecond semiconductor die including: a second semiconductor substrate; asecond dielectric layer over the second semiconductor substrate; and asecond copper pad in the second dielectric layer, the second copper padhaving a second surface opposite to the second semiconductor substrate,wherein: the first dielectric layer is in direct contact with the seconddielectric layer at a bonding interface between the first and secondsemiconductor dies; and the first and second copper pads form aninterconnect between the first and second semiconductor dies, theinterconnect having a conductive buffer material between the first andsecond copper pads, wherein the conductive buffer layer includesaggregates of conductive particles, and wherein the conductive bufferlayer is less dense than the first and second copper pads.
 17. Thesemiconductor die assembly of claim 16, wherein the conductive buffermaterial is porous and includes copper particles.
 18. The semiconductordie assembly of claim 16, wherein at least a first portion of the firstsurface is conjoined to at least a second portion of the second surface.19. The semiconductor die assembly of claim 16, wherein the firstsurface is separate from the second surface, and wherein the firstcopper pad is electrically connected to the second copper pad throughthe conductive buffer material.
 20. The semiconductor die assembly ofclaim 16, wherein either the first surface or the second surface extendspast the bonding interface.